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44

STRUCTURE OF TEETH

CHAP.

of the  nature   of teeth  is  founded.

It has been pointed out
that the scales of the Elas-
inobraneh fishes consist of
a, cap of enamel upon a
base of dentine, the former
being derived from the epi-
dermis and modelled upon
a papilla of the dermis
whose cells secrete the
dentine. The fact that
similar structures arise
within the month (i.e. the
teeth) is explicable when
it is remembered that the
mouth itself is a late in-
vagination from the out-
side of the body, and that
therefore the retention by
its tissues of the capacity
to produce such structures
is not remarkable.

The relations of the
three constituents of the
tooth in its simplest form
is shown in the accom-
panying diagram, where
the intimate structure of
the enamel, dentine, and
cement (or crxista petrosa

widely opeii.at basej III, completely formed   us   it    is   SOniOtlXXXeH   called)

FIG. 33.Diagrammatic sections of various forms
of teeth. J, Incisor or tusk of Klephant,
with pulp cavity persistently open at base ;

is riot indicate*!. The latter
has the closest resemblance
to Tbone. The dentine is
traversed by fine canals

Human incisor, with pxilp cavity opening by
a contracted aperture at base of root; TV,
Human molar with, broad crown . arid two
roots,; V, molar of the Ox, with the enamel
covering1 the crown deeply folded, and the
depressions filled up with cement; the stir-
face is -worn, by xise, otherwise the enamel
coating would be continuous at the top of the Which run, parallel to each
ridges. la all the tigures the enamel is black, o*}ltxr m1 iiTViatoraAMO lnvo
the pxilp white ; the dentine represented by QUIlt'r <iua HUciStOIIXOBO miC
horizontal lines, and the cement by dota. aild. there* The exiatXlol IB
(After Flower anil Lydrtker.)                          formed of long prismatic
fibres, and is excessively hard in strixcturo, coutaiixixig less uixiinul
matter than tho other tooth tisflutw.     To this fact in  frequently